1. Field of the Invention
The present invention relates to a matrix-addressed driving method used in a matrix type flat panel display device utilizing, for example, a bistable display material, such as a ferroelectric liquid crystal.
2. Prior Art
A matrix type flat panel display device has been known, wherein a ferroelectric liquid crystal having fast switching characteristics and bistability (memory property) is used. Various methods of driving such a display device have also been proposed, in which the bistability thereof is utilized. For example, in the two-field method, each frame is comprised of two contiguous fields including a first field for displaying black and holding white picture elements and a second field for displaying white and holding black picture elements. However, since two separate fields respectively for displaying black and white must be defined in this known method, the time T.sub.f (frame period) required for the completion of setting each frame becomes long. Let the pulse width (selection time) required for setting each frame to white or black be 2.tau., the total writing time for the completion of setting becomes 4.tau.. As a result, it becomes necessary to use a liquid crystal which makes it possible to shorten the selection time (or pulse width) 2.tau. to a very short time period. However, it is extremely difficult to obtain a material having such a fast response property as requested. In addition, since the pulse width 2.tau. cannot be decreased, a long frame period T.sub.f becomes inevitable, with the occurrence of disadvantageous flicker problems.
As an alternative, an equi-divided scanning method is also known. In this method, as shown in the illustration of FIG. 18, when it is desired to achieve a multi-graduation display of (n+1) levels (n=15 in the example shown in the Figure), one frame period T.sub.f is divided into n blocks each having an equal width, and scanning is effected along all scanning lines (the number Y of scanning lines being set, for example, to 480) of each divided block in such a manner that the timing for writing for every scanning lines is delayed by a small time lag. The mark RS in the Figure shows simply the timings for writing scans of respective scanning lines. In this known method, the number of blocks which are set to black or white is changed within the range of from 0 to 15 depending on the desired graduation. For example, when it is desired to set a particular frame to graduation 1, the writing scanning is effected for a time period corresponding to the time necessary to set the first block 1 to black or white; whereas when a particular frame is desired to set to be graduation 10, the writing scanning is effected for a time period corresponding to the time necessary for setting the blocks 1 to 10 to black or white.
However, this method has a problem that the writing pulse width 2.tau. for setting each block to white or black should be extremely small. In detail, EQU 2.tau.=T.sub.f /(Y.times.n)=T.sub.f /(480.times.15)=T.sub.f /7200
As a result, it also requires a liquid crystal having an extremely fast response property.
Also known in the art is a simple equi-division frame period shortened scanning method (for example, by Unexamined Japanese Patent Publication No. 62-56936 (corresponding to U.S. Pat. No. 5,011,269 and European Patent Publication No. 214857A)). As will be seen from FIG. 19, in which an embodiment of this known method for achieving a 16(=2.sup.4) graduation display is shown, this known method enables a desired graduation display by dividing the frame period T.sub.f into equal time blocks 1 to 4 and using respective blocks 1 to 4 to correspond to 8, 4, 2 and 1 graduation levels. Writing scanning for each block 1 to 4 is effected at timing RS denoted in the Figure, and for the blocks 2, 3 and 4, reset scanning is effected after the lapse of time corresponding to 4, 2 and 1 graduation level from the writing timing RS to forcibly reset all picture elements along the scanning lines. Thus, the blocks 2, 3 and 4 correspond respectively to 4, 2 and 1 graduation levels.
This method improves in that the pulse width 2.tau. is improved as follows: EQU 2.tau.=T.sub.f /(480.times.4)=T.sub.f /1920
However, this method has a problem that the percent transmission is considerably lowered. For instance, the brightest graduation level 15 is made bright only for a time of (15/32).times.100=47%. This also gives rise to a problem that the contrast of the displayed image is lowered.
To cope with this problem, it has been proposed to shorten the time periods of respective blocks 1 to 4 depending on the graduation level of respective blocks (frame period shortened scanning method). FIG. 20 is an illustration showing the method having a graduation of 16 levels, in which the frame period T.sub.f is divided into 15(=2.sup.4 -1) graduation blocks and each first block of four fields F.sub.1, F.sub.2, F.sub.3 and F.sub.4 respectively having 1, 2, 4 and 8 blocks is used as the writing block for effecting writing scanning denoted by RS. As the result, at the brightest graduation level 15, whole frame period T.sub.f is set to white so that the percent transmission thereof can be brought to 100%.
However, the pulse width 2.tau. necessary for writing must be extremely shortened, since the pulse width 2.tau. takes the following value of: EQU 2.tau.=T.sub.f /(480.times.15)=T.sub.f /7200
However, it is extremely difficult to prepare a liquid crystal having such a fast response property, as described hereinbefore.
To solve this problem, a method has been proposed in which Y scanning lines are divided into plural groups and respective groups are scanned simultaneously and separately (Unexamined Japanese Patent Publication No. 01-61180 (corresponding to U.S. Pat. No. 4,929,058 and European Patent Publication No. 306011A)). In this method, as shown in FIG. 21, the scanning lines Y are divided into M groups, wherein M is the number of combinations in arranging respective fields F.sub.1 to F.sub.4 by which the first writing blocks of respective fields F.sub.1, F.sub.2, F.sub.3 and F.sub.4 do not overlap with each other. For example, for a 16(=2.sup.4) graduation display, since three combinations are considered as shown in the Figure, lines Y=480 are divided into three groups Y.sub.1, Y.sub.2 and Y.sub.3 each having 160 scanning lines and respective groups Y.sub.1 to Y.sub.3 are scanned simultaneously and separately.
According to this method, the pulse width 2.tau. takes the following value of: EQU 2.tau.=T.sub.f /(160.times.15)=T.sub.f /2400
Thus, it is possible to set the pulse width 2.tau. to three times as wide as that in the method shown in FIG. 20. Likewise, since two combinations can be considered for 8 (=2.sup.3) graduation display as shown in FIG. 22, .tau. can be prolonged to two times; and since four combinations can be considered for 32(=2.sup.5) graduation display as shown in FIG. 23, .tau. can be prolonged to four times.
On the other hand, in order to improve the quality of the displayed image, it is desirable to increase the number of combinations of the fields F.sub.1 to F.sub.4, in which the blocks (i.e. writing blocks B(RS) each containing the writing scanning (RS)) are not overlapped with each other, in other words, it is desirable to increase the number of groups Y.sub.1, Y.sub.2, Y.sub.3, - - - of the scanning lines Y shown in FIGS. 21 to 23. However, the prior frame period shortening methods shown in FIGS. 21 to 23 have the problem that it is neither possible to increase the possible combination number M nor to prolong the pulse width (namely, selection time) 2.tau..
On the other hand, it is necessary to prevent flicker of the display image in order to improve the quality of the displayed image. Since flicker occurs when on-off timing of adjacent picture elements are synchronized or close with each other within one frame period T.sub.f, it is desirable that the periodical on-off operations of adjacent picture elements are effected by the longest possible time intervals.